As integrated circuit technology migrates to lower and lower voltages, there is a need to provide a plurality of power supply pins on a printed circuit board (PCB) such that a variety of voltages can be applied thereto. For example in a personal computer, a printed circuit board, typically referred to as an expansion board is plugged into a connector in the motherboard of the personal computer. More particularly, for example, the Peripheral Component Interface (PCI) standard utilized in such computers provides for both 5 volt and 3.3 volt signaling environments.
One goal of such a standard is to provide a quick and easy transition from one process technology to another. In order to facilitate such a transition, the standard defines two expansion board connectors, one for the 5 volt signaling environment, and one for the 3.3 volt signaling environment. This allows for either voltages to be used on this particular type of component,
In the 5 volt signaling environment, all the 3.3 volt and any unused 5 volt pins need to be coupled to the ground plane, to ensure they continue to function as efficient signal return paths. In such a system, all power planes must be decoupled to ground in such a way to provide for reasonable management of the switching currents to which the planes and the supply paths are subjected. Accordingly in such a system, the 3.3 volt pins, even if they are not delivering power, must provide an alternating current return path and must be bussed together on the motherboard preferably on a separate power plane and decoupled to ground in a manner consistent with high speed signaling techniques.
One of the problems associated with the transition of using multiple voltages on a single expansion board utilizing a standard such as described above is that to ensure an adequate return path, there typically must be a plurality of high speed capacitors on the expansion board. The most obvious way to attempt to address this problem is to place the plurality of capacitors and the supply on the expansion board directly on the 3.3 volt plane, thereby ensuring that the specifications are met. However, the problem with this solution is that as the PCB becomes smaller and smaller these capacitors are harder to place on the PCB. Accordingly, placing a large number of capacitors would increase the manufacturability and cost of the PCB significantly.
One approach to solving this problem would be to place the capacitors on the underside of the circuit board, that is, on the side away from where the rest of the components are placed. This approach would add significantly to the overall manufacturing costs of the PCB.
Another approach to solving this problem would be to build the PCB with two additional signal layers. The additional layers would provide extra routing for signal paths blocked by the large number of capacitors. Again, this approach would add significantly to the manufacturing cost of the PCB.
Accordingly, what is needed is a system of ensuring that the interconnections associated with providing adequate return path in the above-identified situation is resolved in a cost effective manner. More particularly, what is needed is a method and system for decoupling of unused 3.3 volt supply pins, for example in a PCI standard on an expansion board, and providing a return path therefore without adding significant cost and expense to the expansion board.
The present invention addresses such a need.